(1) Field of the Invention
The present invention relates to an improvement of an electrolyte of lithium cells.
(2) Description of the Prior Art
Conventional lithium cells can be used in a temperature environment of up to approximately 85° C. However, when lithium cells are incorporated into electrical components of vehicles (air-pressure gauges for tires, on-vehicle devices of the Electronic Toll Collection system, and the like), FA (Factory Automation) appliances, and the like, the cells are often exposed to a harsh temperature environment of over 100 to 150° C.
To enhance productivity, when the cells are incorporated into electronic appliances, the technique of reflow soldering is employed. With this technique, a cell temperature reaches, though only temporarily, as high as 200 to 260° C. In view of this, there is a need for highly reliable lithium cells in heat resistivity that do not swell or do not deteriorate their cell characteristics under such harsh temperature environment.
As a technique to enhance safety of secondary lithium cells, there is proposed a technique in which diethylene glycol dimethyl ether or triethylene glycol dimethyl ether is used as a main solvent of an electrolytic solution (Japanese Unexamined Patent Publication No. H1-281677).
As a technique to enhance the discharge characteristic of secondary lithium cells and to impart high temperature resistivity thereto, there is proposed a technique in which the main solvent of the electrolytic solution is butyl diglyme (diethylene glycol dibutyl ether), which has a high boiling point, and a separator and a gasket used are made of polyphenylene sulfide, which is heat resistant resin (Japanese Unexamined Patent Publication No. 2002-298911).
There is also proposed a technique in which carboxylic acid or carboxylic acid ester is added in a nonaqueous electrolyte (Japanese Unexamined Patent Publication Nos. H8-321311 and H9-147910).
However, with the technique disclosed in H1-281677, heat resistivity is insufficient because the separator and gasket used here are made of low heat-resistant polypropylene (melting point: approximately 150° C.). For this reason, the cells cannot be used in the above fields of application, where a long period of stability against temperatures of near 150° C. is required, and also cannot be used in reflow soldering, where a cell is exposed to temperatures of at least 200° C.
With the technique disclosed in 2002-298911, although heat resistivity is excellent, the viscosity of the nonaqueous electrolytic solution is high because the main solvent is the highly viscous butyl diglyme (diethylene glycol dibutyl ether). This lowers the ionic conductivity of the electrolytic solution, resulting in a poor discharge characteristic.
With the technique disclosed in H8-321311, a cell is provided with a nonaqueous electrolytic solution in which at least one solvent of high dielectric constant selected from the group consisting of ethylene carbonate, propylene carbonate, and butylene carbonate, and 1,2-dimethoxyethane are mixed at a volume ratio of 3:7 to 7:3. However, the solvent of high dielectric constant reacts with the negative electrode under a condition of high temperature and forms a highly resistant coating film on the surface of the negative electrode. This reaction occurs conspicuously in a condition of high temperature, and since the solvent of high dielectric constant is contained at a high ratio of 30 volume percent or higher, the amount of the formed coating layer is excessive. Since internal cell resistance increases due to this coating film, the cell cannot be used in the above fields of application, where a long period of stability against temperatures of near 150° C. is required, and also cannot be used in reflow soldering, where a cell is exposed to temperatures of at least 200° C.
With the technique disclosed in H9-147910, the nonaqueous electrolytic solution used is a nonaqueous solvent in which at least one cyclic carbonic acid ester of high viscosity selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate are mixed at a volume ratio of approximately 1:1. Here, the same problem arises as the technique of H8-321311. Accordingly, the cell cannot be used in the above fields of application, where a long period of stability against temperatures of near 150° C. is required, and also cannot be used in reflow soldering, where a cell is exposed to temperatures of at least 200° C.